China

Tensor network theory and quantum simulation are respectively the key classical and quantum methods in understanding many-body quantum physics. Here we show hybridization of these two seemingly independent methods, inheriting both their distinct advantageous features of efficient representations of many-body wave functions. We introduce the framework of hybrid tensor networks with building blocks consisting of measurable quantum states and classically contractable tensors. As an example, we demonstrate efficient quantum simulation with hybrid tree tensor networks that use quantum hardware whose size is significantly smaller than the one of the target system. We numerically test our method for finding the ground state of 1D and 2D spin systems of up to 8×8 and 4×3 qubits with operations only acting on 8+1 and 4+1 qubits, respectively. Our approach paves the way to the near-term quantum simulation of large practical problems with intermediate size quantum hardware, with potential applications in quantum chemistry, quantum many-body physics, quantum field theory, and quantum gravity thought experiments.

Quantum 2020

Quantum Simulation with Hybrid Tensor Networks

tensor network

quantum many-body physics

quantum simulation

The Institute of Physics and IOP Publishing, in partnership with the Chinese Physical Society (CPS) and the University of Science and Technology of China (USTC) are proud to have brought together the international quantum science community online for this inaugural conference.


Early career and leading researchers from universities, industry and government worldwide came together to share, learn and collaborate on the latest research and emerging areas of high interest across quantum science and technology. The broad themes covered by Quantum 2020 included:

Quantum communication;
Quantum computing and quantum simulation;
Quantum sensing, metrology and imaging;
Quantum information theory, software and algorithms;
Quantum foundations;
Quantum technology in industry.
The four-day event featured talks from world-renowned scientists, interactive poster sessions and two exciting panel discussions on the themes of Quantum Technology in Industry and Quantum Technology Programmes Around the World.


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The Institute of Physics and IOP Publishing, in partnership with the Chinese Physical Society (CPS) and the University of Science and Technology of China (USTC) are proud to have brought together the international quantum science community online for this inaugural conference.

Early career and leading researchers from universities, industry and government worldwide came together to share, learn and collaborate on the latest research and emerging areas of high interest across quantum science and technology. 

technical paper

We consider the estimation of a Hamiltonian parameter of a set of highly photosensitive samples, which are damaged after a few photons Nabs are absorbed. The samples are modelled as a two mode photonic system, where photons simultaneously acquire information on the unknown parameter and are absorbed at a fixed rate. We show that arbitrarily intense coherent states can obtain information at a rate that scales at most linearly with Nabs, whereas quantum states with finite intensity can overcome this bound. The quantum advantage scales like N/Nabs (where N is the number of photons in the quantum state), so that few-photon entangled states can already surpass the classical bound when Nabs is small. We discuss an implementation in cavity QED, where Fock states are both prepared and measured by coupling atomic ensembles to the cavities.

Weakly invasive metrology: quantum advantage and physical implementations

A single quantum measurement cannot perfectly determine the state of a quantum system; however it is possible to perfectly rule out certain states. Quantum state elimination is of foundational interest, relating to the so-called reality of the wave function, as well as having applications in quantum information and quantum communication. We consider unambiguous quantum state elimination for two qubits, where each qubit can be in one of two possible states resulting in four possible two-qubit states. We provide an optimal measurement for eliminating two states out of four two-qubit states. We also explore different possible optical realisations of this state elimination measurement using linear optics and generate a realization that uses a minimum number of optical elements. This state elimination measurement could be used for an XOR oblivious transfer protocol, where a receiver obtains either the first, second, or XOR of two bits a sender sends and the sender not knowing which bit was received. Moreover, one could also envisage a novel quantum key distribution schemes employing state elimination, where the final bit value cannot be thought of as created at the sender and transmitted to the receiver, but is only realised once the receiver's measurement is complete.

Unambiguous quantum state elimination of pairs of quantum states for quantum communication

Quantum coherence of electron motion dramatically affects low temperature phenomena in confined conductors leading to such effects as Aharonov-Bohm oscillations in mesoscopic rings. In Ref. 1, a novel type of the oscillations manifesting itself in finite singly-connected conductors and associating with special diffusive trajectories grazing their boundaries was predicted. We apply this idea to implement SQUID-like oscillations in the critical current dependence on the magnetic field in Josephson junctions with superconducting electrodes and a weak link formed by periodically alternating normal and superparamagnetic ultrathin films, the sample details and the supporting information can be found the supporting information can be found in our paper [2]. We have identified the presence of a universal transfer matrix that maps the incident electron waves to the transmitted waves and is similar to that found before in numerous studies with coherent electromagnetic and acoustic waves passing through a multiply scattering medium. In our case, the presence of almost open conductance eigenchannels reveals itself in a pronounced background of the critical current-vs-field dependence. Possible improvement of quantum magnetic field sensors using the new type of Josephson junctions is discussed. 
1. I.L. Aleiner, A.V. Andreev, V. Vinokur, Phys. Rev. Lett. 114, 076802 (2015).
2. I.P. Nevirkovets, M.A. Belogolovskii, J.B. Ketterson, Phys. Rev. Appl. 14, 014092 (2020). 


Quantum transport in Josephson junctions with a periodic normal metal/superparamagnet weak link

We consider the transport properties of a single-electron transistor consisting of a diffusive normal-metal island and superconducting electrodes. We show that the interplay of a dissipative impedance of the gate electrode, a weak magnetic field, and a charging energy of the island causes a suppression of the critical supercurrent of the transistor. When the device is voltage biased, pumping of single electrons can be achieved by a periodic modulation of the gate voltage [1-3]. We derive the analytic expressions for the error rates and show that they are suppressed by the applied magnetic field [4].

[1] J. P. Pekola et al, Nat. Phys. 4 (2008) 120.
[2] D. V. Averin and J. P. Pekola, Phys. Rev. Lett. 101 (2008) 066801.
[3] S. Nakamura et al, Phys. Rev. Appl. 7 (2017) 054021.
[4] V. Bubanja, Phys. Rev. B 97 (2018) 224516.

Effect of quantum fluctuations on the critical supercurrent through a mesoscopic normal-metal island

The recent miniaturization of heat engines to the nanoscale introduces the possibility of engines that harness quantum resources. The analysis of quantum engines provides important insight into how their efficiency compares to classical analogues and deepens our understanding of thermodynamic mechanisms at the quantum scale. We examine a quantum Otto engine with a harmonic working medium consisting of two particles to explore the use of wave function symmetry as an accessible resource. It is shown that a bosonic working medium displays enhanced performance when compared to distinguishable particles, while a fermionic working medium displays reduced performance. To this end, we explore the trade-off between efficiency and power output and the parameter regimes under which the system functions as engine, refrigerator, or heater. Remarkably, the bosonic system operates under a wider parameter space both when operating as an engine and as a refrigerator.

Bosons Outperform Fermions: The Thermodynamic Advantage of Symmetry

Variational quantum algorithms are believed to be promising for solving computationally hard problems on noisy intermediate-scale quantum (NISQ) systems. Gaining computational power from these algorithms critically relies on the mitigation of errors during their execution, which for coherence-limited operations is achievable by reducing the gate count. Here, we demonstrate an improvement of up to a factor of 3 in algorithmic performance for the quantum approximate optimization algorithm (QAOA) as measured by the success probability, by implementing a continuous hardware-efficient gate set using superconducting quantum circuits. This gate set allows us to perform the phase separation step in QAOA with a single physical gate for each pair of qubits instead of decomposing it into two CZ gates and single-qubit gates. With this reduced number of physical gates, which scales with the number of layers employed in the algorithm, we experimentally investigate the circuit-depth-dependent performance of QAOA applied to exact-cover problem instances mapped onto three and seven qubits, using up to a total of 399 operations and up to nine layers. Our results demonstrate that the use of continuous gate sets may Q2 be a key component in extending the impact of near-term quantum computers.

Improving the Performance of Deep Quantum Optimization Algorithms with Continuous Gate Sets

The theory of the asymptotic manipulation of pure bipartite quantum systems can be considered completely understood: the rates at which bipartite entangled states can be asymptotically transformed into each other are fully determined by a single number each, the respective entanglement entropy. In the multipartite setting, similar questions of the optimally achievable rates of transforming one pure state into another are notoriously open. This seems particularly unfortunate in the light of the revived interest in such questions due to the perspective of experimentally realizing multipartite quantum networks. In this talk, we report substantial progress by deriving simple upper and lower bounds on the rates that can be achieved in asymptotic multipartite entanglement transformations. These bounds are based on ideas of entanglement combing and state merging. We identify cases where the bounds coincide and hence provide the exact rates. As an example, we bound rates at which resource states for the cryptographic scheme of quantum secret sharing can be distilled from arbitrary pure tripartite quantum states. This result provides further scope for quantum internet applications, supplying tools to study the implementation of multipartite protocols over quantum networks.

Rates of Multipartite Entanglement Transformations

Quantum simulation, being one of the most promising applications of a quantum computer, has been realised on current noisy intermediate-scale quantum (NISQ) devices using different algorithms. Among these, the variational quantum eigensolver is extensively used for finding the ground state energy of molecular Hamiltonians. The feasibility and performance of this algorithm depend critically on the number of variational parameters in the wavefunction ansatz and on the depths of the state preparation circuits that create the ansatz on the quantum processor. Recently, an algorithm termed ADAPT-VQE was introduced to build system-adapted ansätze with substantially fewer variational parameters compared to other approaches. However, deep state preparation circuits remain a challenge. Here, we present a hardware-efficient variant of this algorithm called qubit-ADAPT. By numerical simulations on H4, LiH and H6, we show that with a well-designed operator pool, qubit-ADAPT can reduce the circuit depth by one order of magnitude while maintaining the same accuracy as the original ADAPT-VQE. Addressing the high measurement cost which is proportional to the size of the operator pool, we show the possibility to construct a sufficient pool with size growing linearly in the number of qubits. This result highlights the promise of adaptive simulation algorithms on near-term quantum devices.

qubit-ADAPT-VQE: An adaptive algorithm for constructing hardware-efficient ansätze on a quantum processor

The possibility of readout and transferring information through qubits are crucial issue in the development of quantum computers. One of the promising ways to construct qubits is based on single phosphorus atoms doped in crystalline silicon. In present work Quantum Espresso software package, based on the density functional theory and the pseudopotential method, was used to non-collinear study of the equilibrium spin orientation of P atom in silicon matrix and its manipulation possibility. The total magnetization of the Si:P system is determined by the presence of an unpaired electron of the phosphorus atom. Excess electron density is not localized only near the phosphorus atom, but is spatially distributed in some area, so magnetization also has a spatial distribution. We had calculated the local magnetic field B(r), the local magnetization m(r) and the spin current density created by the excess electron of the P atom. It had been shown that the spin current densities has different directions for the |0> and |1> quantum states.

The work was supported in accordance with the State assignment for research work implementation from the Ministry of Science and Higher Education of the Russian Federation (project No 0818-2020-0005).

Ab initio study of possibility of phosphorus atom spin manipulation in doped P:Si crystal

The multiphoton Jaynes-Cummings model is investigated and applications in quantum information science are explored. Considering the strong atom-field coupling regime and an N-photon interaction, a nonlinear driving field can perform an arbitrary rotation in the Fock space of a bosonic mode involving the vacuum and an M-Fock state, with M < N. In addition, driving a bosonic mode with a linear coherent field (superposition of many Fock states), only the cavity states within the Fock subspace |0>; |1>;…; |N − 1> can be populated; i.e., we show how to implement a Fock state filter, or quantum scissor, that restricts the dynamics of a given bosonic mode to a limited Hilbert space. Such a device can be employed as a generator of finite-dimensional quantum-optical states and also as a quantum-optical intensity limiter, allowing as a special case the generation of single-photon pulses. On the other hand, our system also provides a very rich physics in the weak atom-field coupling regime, in particular, multiphoton electromagnetically induced transparencylike phenomena, inducing a narrow (controllable) reflectivity window for nonlinear probe fields. These results are useful for applications in quantum information processing and also motivate further investigations, e.g., the use of an N-photon Jaynes-Cummings system as a qudit with harmonic spectrum and the exploration of multiphoton quantum interference.

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Quantum Simulation with Hybrid Tensor Networks

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